1994A&A...287..523S

A model for rotating pre-main sequence stars is presented. The hypothesis explored here is that the lower mass T Tauri stars (M < 1.5 M_☉), dominated by convection, are in differential rotation with the equator rotating considerably faster than the poles (consistent with the sun). In this context, weak-line T Tauri stars (WTTS), the stand-alone objects, possess surface activity in the form of starspots predominantly at low latitudes (±20deg). In contrast, the classical T Tauri stars (CTTS), influenced by an accretion disc via a strong dipole magnetic field, are spotted only at high latitudes (±60deg). The many consequences are all consistent with the observations: (i) a bimodal distribution of photometric rotation periods with the CTTSs as the slow rotators yet (ii) no equivalent distinction between the spectroscopic periods of the two groups, (iii) the period relationship extends to high-mass T Tauri stars provided they maintain a significant convection zone, (iv) a fundamental difference in the spot temperature between the two classes (v) coronal and chromospheric activity, such as the X-ray emission, directly related only to the inherent stellar properties independent of the environment, (vi) abrupt changes to the light-curve period or the simultaneous presence of two distinct periods and (vii) spots on many CTTSs which are inconsistent with any orientation angle of the rotation axis if rigid rotation is assumed. Further predictions, speculations and problems are discussed based on a general Reynolds stress scheme. The large differential rotation is consistent with the generation of a dipole magnetic field capable of disrupting the innermost part of an accretion disc and directing the flow towards the poles.